The problem of recovery of fine solids from suspensions is complementary to that of recovering clear liquids from suspensions.
A detailed recent discussion of cross-flow microfliltration is given by R. Bertera, H. Steven and M. Metcalfe, The Chemical Engineer, pp. 10-14, June, 1984.
Economically, the ability to cope with strongly fouling solids without filter aids is most pressing. This fouling problem has long been recognised and the art records some attempts to substitute gas for clarified liquid during reverse flow to avoid the recycle of clarified liquid to the feed suspension.
Transmembrane gas reverse flow is impossible in very finely pored filters such as reverse osmosis membranes and ultrafilters because the pressures needed to overcome surface tension are far beyond the strengths of normal hollow fibre membranes used for these purposes; wettable liquids may pass but not gases. Any gas bubbles passing through such a membrane indicate the presence of pin hole defects in the membrane. Hence, this invention has no application to reverse osmosis or to true ultrafilters.
This invention is concerned with microfilters which contain larger pores than those of ultrafilters and which range from 0.001 to 10 microns. Usually, the larger of the pores are so distributed that clarified liquids are free of all visible turbidity. Turbidity of the clarified liquid involves more than pore and particle size, obeying and arising from well known optical laws.
Early microfilters fouled quickly since they treated particles which were not suspended by Brownian motion nor diffusion but which penetrated into the range of similar pore size in the manner of sieve blinding.
One prior art approach to solving this problem was to operate hydrophilic microfilters in a cross-flow mode with clarified liquid transmembrane reverse flow. High crossflow velocities required feed suspension to be directed to the smaller internal filtering surface of the lumen as opposed to the larger external surface of the fibre. Thus, reverse flow pressures had to be limited to avoid fibre crushing. The smaller filtering surface reduced output and this was frequently not a useful solution to the problem.
Another prior art approach is disclosed in Japanese Patent Kokai Publication No. 53-100882 (1978) where a hollow fibre bundle in loose "candle" configuration of hydrophilic "polyvinyl alcohol (PVA)" fibres was made to writhe during long (one minute) lumenal reverse flows with air. Filter "candles" of the kind described in this Japanes specification are more akin to dead-end filters than to cross-flow shell and tube filters in that they are in the form of elongated hollow pots closed at one end.
A development of the above "candle" configuration of hydrophilic "polyvinyl alcohol (PVA) type polymer" porous hollow fibres is to be found in U.K. Pat. No. 2,120,952. The writhing of the fibre bundle was somewhat restrained by taping the fibres loosely and enclosing them in an open sleeve which avoided the tangling and fibre breakage of the earlier mentioned Japanese Kokai Publication No. 53-108882(1978) but the gas reverse flow took 5 minutes.
The prior art in respect of reverse flow cleaning of porous hollow fibres using liquid and gas is very limited and such is also the case in relation to the cooling crossflow sueparators treating hot feed stocks to permit fast operation in the separation mode.
Cooling hot fibres by blowing air on them or by evaporating water from them involves the same thermal principles as shaking a towel soaked in water to cool it; these thermal principles are well known.
However, the problem of using these thermal principles in precise rapid ways to cool hot separating devices to exact temperatures in uniform fashion is difficult. Most attention has been to cooling the air evaporatively by passing the air through a porous packing down which water passes. In general, a temperature gradient forms in the porous material which is undesirable with hollow fibres. On the contrary, the hottest spots of the fibre have the highest vapour pressure and lose temperature fastest, thus reaching uniformity of temperature.